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1994 Silverado with Low Idle (zero - 300 RPM) Problem

  
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1994 Silverado with Low Idle (zero - 300 RPM) Problem

 
Alaska_Chevy Alaska_Chevy
New User | Posts: 2 | Joined: 01/08
Posted: 01/12/08
11:10 PM

Hi,
We are hoping we can get some help from either a chevy technician or someone who has experienced the same problem we are.  We have tried everything we can think of to fix this low idle problem.  

We have a 1994 Chevy Silverado, TBI, 5.7L, 4X4, that has a periodic very low idle (100-200 RPM)while in gear (D) and sometimes when in reverse (R).  

In order to fix this problem, we have replaced PCV, spark plugs, wires, rotor cap, fuel filter, MAP, IAC, TPS, and O2 Sensor.  We have no trouble codes showing, and we used the code reader to check to see if the code was working, and we get a code 12 showing that it is working correctly.  We also cleaned the throttle body assembly and checked all vacuum lines and wire connections.  Everything checks fine.  When replacing the IAC we also cleaned the bore for the IAC. We have done all of this and the truck still has the same low idle problem.

The voltage regulator and alternator is working correctly.  The oil pressure is low only when idle is running low, other than that oil pressure is good.  

The truck runs good except when we stop at red lights and running reverse.  When coming to a stop, right before stopping, the RPM drop almost to zero, the truck shudders, then the RPMS go up to 800 then drops down to 200 - 300.  The truck does not always do this when coming to a stop, just periodically about half the time.  If we put the truck in neutral it will idle right at 650 - 700 RPM.  

We are hoping someone can help us with this.  Help is greatly appreciated. Thanks

Alaska_Chevy  Confused  

waynep712 waynep712
Enthusiast | Posts: 264 | Joined: 01/08
Posted: 01/13/08
12:23 AM

it really sounds like a bad ground wire.. clean the ones from the battery to the body, to the engine and to the frame...

the ecm grounds things to several points on the motor...

usually to the thermostat housing area... or the back of the throttle body area... these are sometimes twisted off or  broken .. each one leads back to the ecm...  they are all important..

it could also have twisted pair wires insulation falling off somewere in the harness...  i have seen that on other cars and trucks...

and the most likely villian..  the throttle shaft bore...

with the engine off... open the throttle with you hand and wiggle it.. if it moves.. around this is the problem...

what goes on if the throttle shaft bore is worn is the tps voltages vary when the throttle closes..  every time it goes back to a differnt spot... the ecm learns what the voltage was when the truck was first started.. if it drops below that voltage it causes strange things to happen....

since you are way up there... i have rebushed the throttle shafts with 3/8 thin will valve guide liners.. for .375" stems...   a letter Y drill .404 is also needed.. the biggest problem is the tips of the throttle blade screws are deformed so they have to had the deformed ends ground off to get the screws out...  and they are a weird metric size... like 2.5mm .060 pitch...  dont quote me on that...

the shaft is removed ... either end of the bore is drilled out  to the letter y size...  short legnths of the guide liners are inserted and the throttle shaft lubed and worked back in...
shaft size .372..
thinwall for .375 stems.. .015+ .015
drill size..404...    k-line does make a special reamer to do this...  i just drill them...


i think this truck needs to be on a scanner first.. watching various voltages...  and verifying stuff with a digital volt meter...watch the voltages the computer see while the problem is happening... this is only done with a scan tool...

here is a link to an earlier chevy truck online manual

http://www.autozone.com/addVehicleId,1095202/initialAction,repairGuide/shopping/vehicleSelected.htm


csk has scanners for 199 from innova model 3120 ..that do old and new stuff...  

55Guy 55Guy
Guru | Posts: 895 | Joined: 08/07
Posted: 01/14/08
01:51 PM

Check the vacuum hose going from the MAP sensor to the throttle body for signs of trouble. When you replaed the TPS, did you set it correctly? There is a slight adjustment on mounting the TPS to the throttle body (I've got a '92 Silverado with the same setup) if it's not adjusted correctly this could also affect idle.

Another possibility, the ignition module or magnetic pickup in the distributor could be going bad. If the motor's not getting enough timing under load, it'll idle too low.

Let's also look more towards the obvious. When was the last time you changed the fuel filter? Fuel pump? And issue with either of these can cause the problem you're having, because at idle under load if the pump isn't making anouhg pressure, or the filter's partially blocked, the motor will idle real low because it's starving for fuel.  

Jafo220 Jafo220
New User | Posts: 2 | Joined: 05/09
Posted: 05/21/09
10:55 AM

Beleieve me, I know your frustrations. My truck is doing the same thing. Deal is it's now getting worse. Same symptoms as original post, but a few things to add. When turning A/C on, engine does not recognise that it is on and won't idle up the engine to compensate causing engine to die immediatley. Also, it will idle fine when the engine is cold, so it's recognising the cold engine and is idling up the motor to warm it up. I've change the idle air control, and TPS sensor. EGR is fine and new coolent sensor. The pickup coil was replaced about 5 yrs. ago and
I also replaced the spark module in the distributor just after the fuel pump was replaced. Thats kind of when all my simptoms started really, after that spark module. My next best guess is E-prom or the entire ECM. I've delt with the throttle body and the throttle shaft seems fine.


Anymore advice? This can't be all that complicated. But it seems to be.  

chriselks chriselks
New User | Posts: 27 | Joined: 12/08
Posted: 05/22/09
09:28 PM

replace the coolant temp sensor. its next to the thermostat  

waynep712 waynep712
Enthusiast | Posts: 264 | Joined: 01/08
Posted: 05/23/09
12:05 AM

one thing that it might be.....    Worn throttle shaft bores...    with the engine off..  reach over and open the throttle a ***  then see if it will wiggle around...  TBI 's have been known to wear the throttle shaft bore a ton...

what happens is the throttle closes to a different position each time...    watch the throttle position sensor closed voltage...
i would bet that in 10 times opening the throttle... when you close it...  it goes to a different voltage every time...

this also will freak out the computer... as it never knows when the throttle is really closed...

the tps voltage should be about 0.75 volt. and go back to the exact same volt reading every time the throttle closes..

the idle air counts should be between 20 and 50... at idle fully warmed...

the coolant temp needs to be  above 185F...

if the throttle shaft bore is really worn.. and a lot are... they can be rebushed by many good carb shops...    

the screws that hold the throttle blades are held in with a hard to find metric size...  i seem to recall 2.5MM X.06 pitch 7 mm long  with a 8 mm pan head...  warning... do not try to take the screws out without grinding off the smashed ends of the screws... the will break off in the throttle shaft .. even though they start to unscrew...  every time.. ruining the throttle shaft forever...

be sure to ask the carb shop if they do this service.. i have sent them in.. only to get them back with cleaned injectors... and new gaskets...  and the excuse.. oh.. we don't rebush throttle bodys anymore.. we broke off too many screws in the throttle shaft...  i could have screamed...   i being a professional... took a 3/8 thin wall valve guide liner... drilled the housing with a letter Y drill...   .404"    and pushed in sections of the valve guide liner .. then worked the throttle shaft back in... with moly graphite assy lube...   put the new screws in.. with the idle stop adjustment all the way back... centered the blades in the bores.. and tightened them...  worked great...

for those who want numbers....   throttle shaft is .372"   thin wall valve guide has .015 walls..   and the letter y drill... .404.. so there should be like .002 but it is always tight... hence the lube...  

just for those people who want to do it themselfs..    most production engine rebuilders will sell you 3/8 thin wall valve guide liners for 2 or 3 bucks each...  they call them K liners in some shops...


the hardest part was finding the screws...  i bought a few from the end of a batch of specials... just to see if they would work...   when i found they worked... some sewing machine repairman had bought up everything small and metric in the place... and i really really really don;t need 10,000 screws...  

Jafo220 Jafo220
New User | Posts: 2 | Joined: 05/09
Posted: 06/29/09
11:32 AM

Well, i took it to a mechanic and they had it for three days. They checked to see if everything was working. All sensors checked out, pickup coil checked out. He found that unhooking the brown wire for timing advance to the distributer along with advancing the timing up manualy and bumping the idle up manualy, engine runs fine. No stalling. I'dles fine, even with A/C engaged. So He recommened re[placing the PCM. Went up got the PCM, installed it with the PROM and plugged the brown wire back in after I reset the timing and idle. Same stuff all over again. New PCM did'nt change anything. So I went back to unplugging the brown wire, advanced the timing back up along with the idle. Runs great! Just have that "service engine soon" light on the whole time because of the brown wire.


Any more ideas? Throttle body shaft has very minimal play. So I'm sure thats not it.

Jon.